The drilling of oil and gas wells by the rotary technique involves the circulation of a drilling fluid through the drill string, out the bit nozzles and its return to the surface via the annulus. This fluid cools and lubricates the drill string, develops sufficient hydrostatic head to counterbalance formation pressures and removes the cuttings from the borehole. This fluid also helps reduce the frictional forces between the drill string and the borehole or casing.
During the drilling operation, the drill string may become stuck and cannot be raised, lowered or rotated. There are a number of mechanisms possible which may contribute to this problem. Namely these are (1) cuttings or slough build-up in the hole; (2) an undergage hole; (3) key-seating; and, (4) differential pressures.
Differential sticking may be defined as the sticking of the drill string against a permeable formation containing less pore fluid pressure than the hydrostatic pressure exerted by the drilling fluid column and usually occurs when the drill string remains motionless for a period of time. The mechanism by which this occurs involves the drill string coming into contact with the permeable zone, remaining quiescent for a period of time sufficient for mud cake to build-up on each side of the point of contact, thus sealing the pipe against the borehole. The annular pressure exerted by the drilling fluid then holds the pipe against the borehole or the permeable zone.
Freeing of differentially stuck pipe is essentially a matter of reducing this pressure differential which exists across the pipe. One method used simply involves a reduction in fluid pressure by replacing the annular fluid with a less dense fluid allowing for less pressure differential to exist between the borehole and annulus. In some cases, the borehole pressure may exceed the annular pressure which in turn allows the pipe to be blown away from the borehole.
The most commonly used method to release stuck pipe is the spotting of an oil mud in the hole opposite the stuck interval. With time, the isolated area between the drill pipe and borehole is reduced by the oil mud. Too, oil invasion into the mud cake reduces the adhesive forces and lubricates the area between the pipe and borehole resulting in less friction and quicker release. More often than not, an extensive period of time is necessary for this to occur which results in an expensive loss of rig time.
In recent years, there has been a number of proprietary formulations developed aimed at releasing differentially stuck pipe, which formulations include: petroleum oil containing a material selected from the group consisting of 2-heptadecenyl-4-4-di-methyoxy-2-oxazoline, 2-heptadecenyl-4-methoxy-4-methyl-2-oxazoline, the sodium salt of dioctyl sulfosuccinate, a mixture of a product containing high molecular weight free fatty acids, esters and alcohols made by acid cracking wool grease with about one-fourth its weight of the sodium salt of sulfated oleyl alcohol and mixtures thereof, in quantity sufficient to reduce interfacial tension at an interface between the petroleum oil and water to not more than 2 dynes per centimeter (U.S. Pat. No. 3,217,802); a surface-active agent prepared from about four parts of half ammonium half isopropylamine salt of the sulfosuccinic acid ester of the oleic acid amide of isopropanol amine and about one part of the isopropylamine salt of dodecyl benzene sulfonic acid which agent is added to the water base drilling fluid (U.S. Pat. No. 3,233,622); and, a polyethylene glycol having a molecular weight ranging from 106 to about 600 or a saturated salt water solution or a sea water solution of said glycol (U.S. Pat. No. 4,230,587).
There remains a serious need for chemical compositions which can better release stuck drill pipes, particularly those which have improved efficacy for releasing differentially stuck pipe.